Cooling towers of the evaporative type are used to cool water to levels that approach ambient air temperatures. Because the water discharged from cooling towers may be recycled back to the cooling tower from processes that apply the water for cooling purpose, cooling towers are most generally used for the purpose of conserving water resources. Common applications for the cooling water produced in cooling towers include removing heat from critical components or controlling the operating temperature of engines, electrical generators, refrigeration compressors (e.g., air conditioning) and a broad range of industrial processes across a broad spectrum of industries. Natural draft and mechanical draft cooling towers are the two types of cooling towers that are most commonly used in commercial service.
As evaporative coolers, natural draft and mechanical draft cooling towers rely on the transfer of both sensible at latent heat between water and air to cool water by rejecting heat to air. Sensible heat is transferred primarily by conduction as heat flows from the warmer water to the cooler air, and latent heat is transferred through evaporation of a portion of the water into the air. The evaporative effect provides the preponderance of heat transfer in evaporative cooling processes, typically about 80%. Theoretically, heat exchange between the water and air within the cooling tower could continue until the air becomes saturated with moisture (i.e., reaches 100% relative humidity) at the adiabatic saturation temperature of the cooling system. Thus, the lowest theoretical temperature that the water can be cooled to is limited by the inlet temperatures of the air and water and the moisture content (humidity) of the inlet air. In practice, the outlet water temperature from a cooling tower may generally be brought to within approximately 5 degrees Fahrenheit of the wet bulb temperature of the incoming air stream.
Because cooling towers rely on heat and mass transfer between water and air and both conductive heat transfer and evaporation proceed fastest when the two phases are in direct contact, the efficiency of a coo ting tower is critically dependent on bringing the air and water into intimate contact over a large area of interfacial surface between the two phases. Likewise, given that intimate contact over a finite interfacial surface area exists, increasing the degree of mixing or turbulence within the water and air phases increases both the rate of conductive heat transfer and evaporation (mass transfer). However, the particular method used to create interfacial surface area and the means by which the air is forced to flow over the surface area affect the size of the cooling tower and compact designs are generally more desirable considering: the value of space within industrial facilities; the fact that many cooling lowers are installed on rooftops; and the negative aesthetic impact of larger units.
Natural draft cooling towers are used almost exclusively to cool large power generating plants due in part to the tremendous size of such towers and the high circulating rates of the cooling water required to justify the costs of building the towers, usually in range of 100,000 to 200,000 gal/min. Natural draft cooling towers use very large concrete chimneys to introduce air into the system and may be 400 feet or more in height for large applications such as providing cooling water for nuclear power plants. Packing or fill located within the chimney of a natural draft cooling tower is typically used to create a large amount of extended surface area for contacting the water and air phases within the chimney. A water distribution system, usually spray nozzles or a weir system, is normally used to uniformly distribute the incoming warm water over the top of the packing or fill. Thus distributed, the water flows downward over the extended surface area within the chimney of the natural draft cooling tower as a thin film under the force of gravity. Air within the chimney that is contact with the surface of the warm water picks up both heat and moisture from the water causing the density (i.e., weight per unit volume) of the air to decrease. This decrease in density creates a buoyancy effect that causes the air to rise upward through and out of the top of the chimney while higher density ambient air is drawn into openings located at the bottom of the chimney. This flow (natural draft) through the chimney of the natural draft cooling tower continues for as long at the driving forces of differential temperature between warm water and cooler air and the ability of the air to absorb moisture persist within the chimney.
Mechanical draft cooling towers, on the other hand, generally use fans to force air over circulated water as the water flows downward under the force of gravity over the same types of packing or fill as used in natural draft cooling towers. Mechanical draft cooling towers are scalable and generally range in height from approximately 10 feet to more than 40 feet, and from a few gallons of cooling fluid per minute to hundreds or thousands of gallons of cooling fluid per minute depending on the particular cooling requirements. Because mechanical draft cooling lowers rely on fans to force air to flow over the film of water distributed over the extended surface area of the packing or fill, the rates of heat and mass transfer per unit of surface area can be increased based on the ability of the fans to directly induce greater turbulence to the air stream and thereby indirectly induce greater turbulence to the water through disturbance of the surface of the water film by the air flowing over it. Thus, at the cost of using higher pressure or higher volume fans, mechanical draft cooling towers can achieve higher efficiency per unit of interfacial contact area than that of natural draft cooling towers. Due to improved efficiency, mechanical draft cooling towers are generally more compact than natural draft cooling towers per unit of cooling, but more expensive to purchase, operate and maintain per unit of cooling.
Of course, typical cooling towers have inefficiencies. Some entrained droplets of water escape with the air as it rises through the cooling tower. This phenomenon is known as drift or windage. To mitigate this problem, known systems employ drift eliminators, typically in the form of a series of baffle-like devices to collect the entrained droplets and return them to the water phase. Additionally, the evaporation of water causes dissolved solids within the cooling fluid to become more concentrated. Eventually, without corrective action, the solids will begin to precipitate out of the cooling fluid and join with additional particles of solids that are brought into the cooling tower with the air causing fouling of the system.
Each of these problems requires mitigation in the form of periodic cleaning and replacement of the concentrated water with fresh water in order to maintain dissolved solids at acceptable levels. Normally, in order to maintain the level of concentration within the cooling tower at acceptable levels, a portion of the cooling water is continuously withdrawn as waste (blow down) and an equal portion of fresh water is continuously added to the cooling tower.
Submerged gas evaporators, also known as submerged gas reactors and/or combination submerged gas evaporator/reactor systems in which gas is dispersed within the liquid phase are used to concentrate wastewater streams by evaporation prior to disposing of any unwanted compounds within the wastewater. Such submerged gas evaporators may be useful as cooling towers as well. U.S. Pat. No. 5,342,482, which is hereby incorporated by reference, discloses a common type of submerged combustion gas evaporator, in which gas is delivered though an inlet pipe to a dispersal unit submerged within the liquid to be evaporated. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each, of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within the processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the evaporation vessel.
Because submerged gas evaporators disperse gas into a continuous liquid phase, for a given ratio of gas to liquid at a particular pressure the volume of the required space within the equipment that is used to bring the two phases into contact is the minimum possible and generally a much smaller volume than that required in gas-liquid contacting devices used in conventional cooling towers where the gas is the continuous phase and the liquid is dispersed into the gas stream either as droplets or thin moving films flowing over the extended surfaces of packing or fill.
However, during the evaporation process dissolved solids within the liquid phase become more concentrated often leading to the formation of precipitates that are difficult to handle. In certain cases precipitation of solids can lead to the formation of large crystals or agglomerates that can block passages within processing equipment, such as the gas exit holes in the system described in U.S. Pat. No. 5,342,482. Generally speaking, liquid streams that cause deposits to form on surfaces and create blockages within process equipment are called fouling fluids
Unlike conventional cooling towers where heat and mass are transferred from the liquid phase as it flows over the extended surface of fill or packing, heat and mass transfer within submerged gas evaporators takes place at the dynamic renewable interfacial surface area of a discontinuous gas phase dispersed within a continuous liquid phase and there are no solid surfaces upon which deposits can accumulate.
Submerged gas evaporators also tend to mitigate the formation of large crystals because dispersing the gas beneath the liquid surface promotes vigorous agitation within the evaporation zone, which is a less desirable environment for crystal growth than a more quiescent zone. Further, active mixing within the evaporation vessel tends to maintain precipitated solids in suspension and thereby mitigates the formation of potential blockages that are related to settling and/or agglomeration of suspended solids.
However, mitigation of crystal growth and settlement is dependent on the degree of mixing achieved within a particular submerged gas evaporator, and not all submerged gas evaporator designs provide adequate mixing to prevent large crystal growth and related blockages. Therefore, while the dynamic renewable heat transfer surface area feature of submerged gas evaporators eliminates the potential for fouling fluids and/or precipitates to coat extended surfaces, conventional submerged gas evaporators are still subject to potential blockages and carryover of entrained liquid within the exhaust gas flowing away from the evaporation zone.
Regardless of the type of submerged gas evaporator, in order for the process to continuously perform effectively, reliably and efficiently, the design of the submerged gas evaporator must include provisions for efficient heat and mass transfer between gas and liquid phases, control of entrained liquid droplets within the exhaust gas, mitigating the formation of large crystals or agglomerates of particles and maintaining the mixture of solids and liquids within the submerged gas evaporation vessel in a homogeneous state to prevent settling and agglomeration of suspended particles.